Summer students in the field

Summer students investigating a host tree with an Ochrogaster lunifer nest

 

The University of Queensland had three enthusiastic summer students on board for this processionary caterpillar research. The summer students aided with collecting Ochrogaster lunifer egg masses and recording the development of nests. Collected egg masses were examined under the microscope for any signs of parasitism by Anastatus wasps. Our lab would like to thank the summer students for their hard work both in the field and laboratory. Click here for more student opportunities.

New season of Bag-shelter moths and processionary caterpillars has started

Tree-hugger and ground nesting Ochrogaster lunifer (Bag-shelter) moths are emerging from their over-wintering places in the ground, and mated females are laying eggs on host trees. The eggs are laid on the bark and then covered with scales from the female’s body. The egg masses (photos below) look like small cotton wool balls because of the covering of scales. The scales are very urticating (itchy), and if they come in contact with your skin you may instantly get a rash and an allergic reaction.

White egg mass of a ground-nesting form of Ochrogaster lunifer (the Bag-shelter moth) laid at the base of the trunk on an Acacia salicina host tree.
Egg mass of the tree-hugger nesting form of O. lunifer laid on a twig in the canopy of an Acacia salicina (which is not normally a host of this form). Tree-hugger, trunk and canopy -nesting forms of the Bag-shelter moth have golden-coloured egg masses.
Newly emerged female tree-hugger moth with a piece of pupal case attached to her wing

It will take approximately 3-4 weeks for an egg to develop and hatch into a first instar larva.

NOW is a good time to search for egg masses and destroy or remove them before they develop into a nest of caterpillars. We have found a sharp chisel is effective at scraping egg-masses from trunks, any remaining eggs can be picked off with tweezers. Egg masses in the canopy can be easily removed with pruners. Don’t remove egg masses in windy weather. Cover up completely and particularly protect eyes from the scales covering the eggs. Work upwind even in light breeze conditions. Once removed, egg masses need to be contained and buried or completely incinerated in an appropriate facility so the scales do not contaminate the environment. Wash all implements well.

Acacias (wattles) and some species of Eucalypt and Corymbia (gum) trees are the main host trees to check.

 

See our gallery of images for more photos of moths and egg masses here.

Pupal stage has started

Diapausing caterpillars are now pupating. When a pupa emerges from its cast skin, it appears green and later darkens to a reddish brown (photo below). The pupa transitions into an adult moth in 2-4 weeks time. The moths will mate and females will deposit her eggs on a host tree.

Freshly emerged pupa with cast skin attached at the posterior
Ochrogaster lunifer pupae                         (female on the left and male on the right)

 

Diapausing caterpillars

Two months have passed since the ground nesting caterpillars left their nests. Although the caterpillars leave the nest in a procession, they break into smaller processions and eventually split off individually or into smaller groups to find a suitable resting spot. These caterpillars can rest underground for approximately 4 months without feeding.

This caterpillar was found 4cm below ground. It will soon spin a cocoon to start its transformation from caterpillar to moth, known as pupation.

Caterpillar processions

Continuing on from the last post, the caterpillars are leaving their nest to over-winter underground and pupate . Caterpillars leave the nest at dawn, forming a procession that can be several meters long.

Caterpillar procession over 3m long

Some caterpillars split off from the main procession, forming smaller processions. Moths will start emerging from the underground pupae from October until approximately end of November.

These caterpillars appear pink because we have applied pink fluorescent dust on them to improve visibility at night, video of the caterpillars feeding at night can be viewed here.

First caterpillar leading the cohort to a suitable place to over-winter and pupate.

High risk of EAFL

Ground nesting procesionary caterpillars are active and on the move. This period from March to June is known to be the high risk period of EAFL. It is time when caterpillars leave the nest and travel (up to 200 metres) to find a suitable place to rest underground (to pupate: developmental process to become a moth). Therefore, during this time the mares may accidentally ingest caterpillar setae (microscopic hairs) that are known to cause EAFL.

Caterpillars are usually well hidden in their nest. However, when they are at their last caterpillar stage (8th instar) they become more visible (see photo below). When you see ground nests where caterpillars are sitting on top of their frass (like the photo below), it is an indication that they will soon leave the nest.

Large ground nest with 8th instar larvae

For details on how to prevent EAFL please click here.

In the field, we have been dusting the caterpillars and inside the nest with fluorescent dust. This dust is non-toxic and it helps us track the movement of caterpillars.

Dusting the inside of a ground nest
Caterpillars marked with pink fluorescent dust

Tree-hugger processionary caterpillars will remain in their nest longer than ground nesting forms. Therefore, the risk period of EAFL is approximately until end of June. Exuviae (shed exoskeleton), dead caterpillars and other nest material are often bursting out from the nest (see photo below) that is usually high up in the tree. These caterpillar remains may become airborne and dispersed further into the environment.
Please be cautious and do not stand underneath the nests.

Tree-hugger nest with live caterpillars, dead caterpillars, frass and exuviae bursting out from the bottom.

 

Caterpillars are growing and attracting parasitoids

The caterpillars are growing and are approximately at their 6th instar stage. Seeing a whole cohort of caterpillars outside its nest during the day like this (see photo below) is unusual, so we had to have a closer look!

A whole cohort of Ochrogaster lunifer larvae outside of its tree-hugger nest.

The last post featured a parasitic Tachinid fly egg attached on the head of a caterpillar. This time, we found a different fly species laying 1st instar maggots directly on the caterpillar’s body! Some caterpillars were weak/immobile because a fly maggot was growing inside the caterpillar’s body. Once the maggot is ready to pupate, it bursts out of the caterpillar’s body and forms a pupa. After a few days, a fly will emerge from the pupa and the life cycle will restart.

Parasitic fly looking for its next victim

Tree-hugger processionary caterpillars

On our last field trip, we found developing tree-hugger nests on the host tree, Corymbia tesserllarisTree-hugger nests look different to ground nests, they form a leathery skin on the tree trunk (see photo below).

Developing tree-hugger nest on Corymbia tessellaris

Interestingly we found a couple of caterpillars above their nest which is unusual, since they rest inside the nest during the day. One caterpillar had a tachinid fly egg laid on its head (see photo below). Tachinid flies are parasitoids of processionary caterpillars, to read more about them click here.

Tree-hugger nest processionary caterpillars, one with tachinid fly egg on its head.

Processionary caterpillars have started building their nest in SE Qld

Caterpillars are starting to hatch from egg masses and ground nests are starting to form.

Newly developed ground nest

Caterpillars from separate egg masses laid close together are likely to join to become one big nest (see photo below). This nest was formed from 3 egg masses. It is common to find egg masses/nests on previously infested host trees (you can tell by the caterpillar frass (poo) and shed exoskeleton surrounding/underneath the current nest).

Developing ground nest on top of the previous season’s nest.

We have also found a few cases of ground nesting caterpillar egg masses laid in the tree fork which is uncommon since they usually lay on the tree trunk (see photos below). This egg mass has successfully developed into a nest above ground!

Ground nest developing on tree fork
Close up of the ground nest